The present invention relates to traveling wave tubes and particularly to traveling wave tubes that can be economically manufactured to provide amplification at low to medium power levels.
Conventional traveling wave tubes utilize a slow wave structure through which an electron beam passes. In the tube, the electrons in the beam travel with velocities slightly greater than that of an r.f. wave, and on the average are slowed down by the field of the wave. A loss of kinetic energy of the electrons appears as increased energy conveyed to the field of the wave. The traveling wave tube may be employed as an amplifier or an oscillator.
Conventional traveling wave tubes employ periodic permanent magnets all along the electron beam to focus the electron beam. They also employ a ceramic-metal brazed construction with sometimes hundreds of ceramic and metal parts fitted and brazed together by skilled artisans. Consequently expense is very high. While this expense appears to be justified at high output power levels, at low output power the cost per watt renders the device economically unfeasible for many purposes. Thus, despite many advantages of the traveling wave tube (high bandwidth, high power, high frequency), it is sometimes replaced by solid state amplifiers at low power levels, say 5 to 100 watts.
In summary much of the expense is attributable to the ceramic-metal-brazed assembly technique and the use of dozens of periodic permanent magnets for focusing. If these were eliminated, tube cost would be dramatically reduced. It would appear that another form of focussing such as electrostatic focussing could be an alternative. However, attempts at providing electrostatic focussing in traveling wave tubes have not heretofore resulted in a practical device.
In accordance with the present invention, a substantially unitary structure comprising an electron gun, a collector and an intermediate slow wave structure is supported on a plurality of substantially parallel glass rods which are themselves disposed within an elongated cylindrical glass envelope. The electron gun and the collector may comprise a series of conductive wafers having pins embedded in the glass rods and apertures to pass the electron beam. Differing voltages are applied to alternate conducting members in the slow wave structure to provide focussing, while r.f. input and output means are located proximate the beginning and end of the slow wave structure for supplying the input r.f. energy and withdrawing the amplified output. The glass rodded structure is economically constructed and maintains excellent alignment for the passage of the electron beam.
In one embodiment, the slow wave structure comprises a ladder circuit within which r.f. energy is propagated back and forth across the electron beam.
In another embodiment, a plurality of r.f. cavities are disposed along the path of the electron beam.
In yet another embodiment, the slow wave circuit comprises a double helix supported by dielectric fins in turn provided with means for attaching the same to the glass rods.
In another embodiment, the slow wave structure comprises a double, interleaved ring loop structure supported by dielectric fins having means for attaching the same to envelope enclosed glass rods.
It is accordingly an object of the present invention to provide an improved traveling wave tube operable at relatively low power levels and providing substantial amplification.
It is another object of the present invention to provide an improved traveling wave tube of economical construction.
It is a further object of the present invention to provide an improved traveling wave tube utilizing electrostatic focussing but characterized by low beam losses in operation.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.